1. Field of the Invention
This invention relates to the deposition of layers of materials on substrates used in the manufacture of semiconductor integrated circuits. More particularly, the invention relates to controlling the deposition of materials at the edges of semiconductor substrates.
2. Background
Chemical vapor deposition, commonly referred to as "CVD," is one of a number of processes used to deposit thin layers of material on a semiconductor substrate. To process deposit substrates with the CVD process, a vacuum chamber is provided with a susceptor configured to receive a substrate thereon. In a typical prior art CVD chamber, the substrate is placed into and removed from the chamber by a robot blade and is supported by the susceptor during processing. Prior to processing, however, the susceptor and the substrate are heated to a temperature of between 250.degree.-650.degree. C. Once the substrate is heated to an appropriate temperature, a precursor gas is charged to the vacuum chamber through a gas manifold typically situated above the substrate. The precursor gas reacts with the heated substrate surface to deposit the thin material layer thereon. As the gas thermally reacts to form the material layer, volatile byproduct gasses are formed, and these gasses are pumped out of the vacuum chamber through a chamber exhaust system.
A primary goal of substrate processing is to obtain as many useful die as possible from each substrate. Many factors influence the processing of substrates in the CVD chamber and affect the ultimate yield of die from each substrate processed therein. These factors include processing variables, which affect the uniformity and thickness of the material layer deposited on the substrate, and contaminants that can attach to a substrate and contaminate one or more die therein. Both of these factors must be controlled in CVD and other processes to maximize the die yield from each substrate.
One of the causes of particulate contaminants in the chamber is improper deposition at the edge of the substrates. Because edge deposition conditions are difficult to control, due in part to the fact that substrates edges are typically chamfered and deposition gas flow is non-uniform around these edges, non-uniform deposition can occur around a substrate's edge. This may lead to deposited layers not adhering properly to each other and/or not adhering properly to the substrate.
This problem is illustrated in FIG. 1(a) which is a schematic partial cross-section of a semiconductor substrate. The substrate 1 is shown with three consecutive layers 2, 3 and 4 deposited thereon. In the deposition of tungsten on the substrate (using WF.sub.6 gas) the first layer 2 could typically be titanium, the second layer 3 would be titanium nitride and the third (upper) layer 4 would be tungsten.
Such a three-layer process for the deposition of tungsten is common as tungsten does not readily adhere to the silicon (or oxidized silicon) surface of the substrate. Accordingly a very thin "primer" layer 2 of titanium is deposited, followed by a second layer 3 of titanium nitride. Tungsten readily adheres to titanium nitride (TIN). As can be seen from FIG. 1(a), however, the tungsten layer 4 has "wrapped" around onto the beveled outer edge 5 of the substrate to contact directly with the silicon substrate.
The problem with this is that tungsten does not adhere to the silicon substrate surface and could readily chip and flake, during the handling of the substrate, resulting in particulate contaminants.
An idealized edge cross-section is, therefore, that illustrated in FIG. 1(b) in which all three layers terminate at the same or close to the same point with respect to the substrate's edge with the tungsten layer 4 being the furthest back from the edge of the substrate.
One solution to this problem is to provide a shadow ring which is located over and masks a narrow, peripheral area of the substrate to prevent deposition thereon. This, however, has the disadvantage that the ultimate yield by the substrate is reduced because its usable area is smaller. It is also inappropriate to use a shadow ring in situations where the entire upper surface of the substrate must be deposited on.
The need therefore exists for a method and apparatus for controlling the deposition of materials at or around the edge of a semiconductor substrate during CVD and/or other substrate processing operations.